1. Field of the Invention
The invention relates to flexible films of cycloolefin copolymers, a process for the production of such films and their use as capacitor dielectrics, thermoformed materials, packaging materials and electrical insulation.
2. Description of Related Art
Cycloolefin copolymers have the following properties that are advantageous specifically for films:
high transparency (for optical uses) PA1 good dielectric properties (as a capacitor dielectric) PA1 high softening points, in particular, at a high cycloolefin content (for high temperature uses) and PA1 good gas barriers (for uses in the packaging sector).
Films prepared from cycloolefin copolymers are known. DD-A 224 538 and DD-A 241 971 describe cast films of ethylenenorbornene copolymers. EP-A 0 384 694 also describes films of cyclic copolymers. Except for the norbornene copolymers, other cycloolefinic starting materials mentioned therein are comparatively expensive and therefore uneconomical.
One disadvantage of many of the known cycloolefin copolymer films is the high brittleness. Films having high brittleness are difficult to handle since these films break easily during winding up and unwinding. This problem is exacerbated for the exceptionally thin films for capacitor dielectrics where the capacity of the capacitor is inversely proportional to the square of the film thickness. Extremely thin films prepared from the known cycloolefin copolymers can not be handled appropriately for the aforementioned uses.
Some cycloolefin copolymers that may be suitable for the production of the films according to the invention are described in EP-A 0 407 870, EP-A-0 503 422 and in DE-A 40 36 264 the disclosures of which are incorporated by reference herein. These documents also mention that films can be produced from these raw materials. These documents also include, however, many cycloolefin copolymers that are not suitable for the production of films according to the invention since they are too brittle and therefore do not have an adequate flexibility. Cycloolefin copolymers having a specific microstructure typically are needed to prepare films having the requisite thickness and flexibility. Suitable mechanisms for preparing cycloolefin copolymers having this specific microstructure described below, however, are not described in these documents.